The present invention is directed to graft copolymers of poly(ethylene oxide) and at least one nonpolar monomer. The graft copolymers of poly(ethylene oxide) and nonpolar monomer described herein are useful as water-soluble adhesives and as components of water-soluble adhesives.
Disposable personal care products such as pantiliners, diapers, tampons etc. are a great convenience. Such products provide the benefit of one time, sanitary use and are convenient because they are quick and easy to use. However, disposal of such products is a concern due to limited landfill space. Incineration of such products is not desirable because of increasing concerns about air quality and the costs and difficulty associated with separating such products from other disposed, non-incineratable articles. Consequently, there is a need for disposable products which may be quickly and conveniently disposed of without dumping or incineration.
It has been proposed to dispose of such products in municipal and private sewage systems. Ideally, such products would be flushable and degradable in conventional sewage systems. Products suited for disposal in sewage systems and that can be flushed down conventional toilets are termed xe2x80x9cflushablexe2x80x9d. Disposal by flushing provides the additional benefit of providing a simple, convenient and sanitary means of disposal. Personal care products must have sufficient strength under the environmental conditions in which they will be used and be able to withstand the elevated temperature and humidity conditions encountered during use and storage yet still lose integrity upon contact with water in the toilet. Therefore, a water-disintegratable material having mechanical integrity when dry is desirable.
Due to its unique interaction with water and body fluids, poly(ethylene oxide) (hereinafter PEO) is currently being considered as a component material for water-disintegratable films, fibers, and flushable products. PEO,
xe2x80x94(CH2CH2O)nxe2x80x94
is a commercially available water-soluble polymer that can be produced from the ring opening polymerization of the ethylene oxide. 
Because of its water-soluble properties, PEO is desirable for flushable applications. However, there is a dilemma in melt processing PEO. Low molecular weight PEO resins have desirable melt viscosities and melt pressure properties for melt processing but have limited solid state properties when melt processed into structural articles such as films.
A commercial example of a low molecular weight PEO resin is POLYOX(copyright) WSR N-80 PEO which is commercially available form Union Carbide. POLYOX(copyright) WSR N-80 PEO has an approximate molecular weight of 200,000 g/mol as determined by Theological measurements. As used herein, low molecular weight PEO compositions are defined as PEO compositions with an approximate molecular weight of less than and including about 200,000 g/mol.
In personal care product industry, flushable thin-gauged films and melt-spun fibers are desired for commercial viability and ease of disposal. The low melt strength and low melt elasticity of low molecular weight PEO limit the ability of the low molecular weight PEO to be drawn into films having a thickness of less than about 1.25 mil. Although low molecular weight PEO can be thermally processed into films, thin-gauged films of less than about 1 mil in thickness cannot be obtained due to the lack of melt strength and melt elasticity of the low molecular weight PEO. Efforts have been attempted to improve the processability of PEO by blending the PEO with a second polymer, a copolymer of ethylene and acrylic acid, in order to increase the melt strength. The PEO/ethylene acrylic acid copolymer blend is able to be processed into films of about 1.2 mils in thickness. However, the blend and resulting film are not water-soluble, especially at high levels of ethylene acrylic acid copolymer, i.e. about 30 weight percent. More importantly, thin films made from low molecular weight PEO are too weak and brittle to be useful for personal care applications. Low molecular weight PEO films have low tensile strength, low ductility, and are too brittle for commercial use. Further, films produced from low molecular weight PEOs become brittle during storage at ambient conditions. Such films shatter and are not suited for commercial applications.
High molecular weight PEO resins are expected to produce films with improved mechanical properties compared to films produced from low molecular weight PEO resins. An example of a high molecular weight PEO is POLYOX(copyright) WSR 12K PEO which is commercially available from Union Carbide. POLYOX(copyright) WSR 12K PEO has a reported approximate molecular weight of 1,000,000 g/mol as determined by Theological measurements. As used herein, high molecular weight PEOs are defined as PEOs with an approximate molecular weight of greater than and including about 300,000 g/mol.
High molecular weight PEOs have poor processability due to their high melt viscosities and poor melt drawabilities. Melt pressure and melt temperature are significantly elevated during melt extrusion of high molecular weight PEOs. During extrusion of high molecular weight PEOs, severe melt fracture is observed. Only very thick sheets can be made from high molecular weight PEOs. High molecular weight PEOs cannot be thermally processed into films of less than about 3-4 mil in thickness. High molecular weight PEOs suffer from severe melt degradation during extrusion and melt processing. This results in breakdown of the PEO molecules and formation of bubbles in the extrudate. The inherent deficiencies of high molecular weight PEOs make it impossible to utilize high molecular weight PEOs in film applications. Even the addition of high levels of plasticizer to the high molecular weight PEOs do not improve the melt processabilities sufficiently to allow the production of thin films without melt fracture and film breakage occurring. In addition, the use of plasticizer in films causes latent problems due to migration of the plasticizer to the film surface.
There is also a dilemma in utilizing PEO in the fiber-making processes. PEO resins of low molecular weights, for example 200,000 g/mol have desirable melt viscosity and melt pressure properties for extrusion processing but cannot be processed into fibers due to their low melt elasticities and low melt strengths. PEO resins of higher molecular weights, for example greater than 1,000,000 g/mol, have melt viscosities that are too high for fiber-spinning processes. These properties make conventional PEOs difficult to process into fibers using conventional fiber-making processes.
PEO melt extruded from spinning plates and fiber spinning lines resists drawing and is easily broken. PEO resins do not form thin diameter fibers using conventional melt fiber-making processes. Conventional PEO resins can only be melt processed into strands with diameters in the range of several millimeters. Therefore, PEO compositions with appropriate melt viscosities for processing fibers and with greater melt elasticities and melt strengths are desired.
In the personal care industry, flushable melt-spun fibers are desired for commercial viability and ease of disposal. PEO fibers have been produced by a solution spinning process. However, it has not been possible to melt process PEO fibers using conventional fiber making techniques such as melt spinning. Melt processing techniques are more desirable than solution casting because melt processing techniques are more efficient and economical. Melt processing of fibers is needed for commercial viability. Prior art PEO compositions cannot be extruded into the melt with adequate melt strength and elasticity to allow attenuation of fibers. Presently, fibers cannot be produced from conventional PEO compositions by melting spinning. There is a need for water-soluble, constructive adhesive compositions comprising PEO that can be used to bond components of flushable articles such as films, non-woven webs elastics, etc. Water-soluble adhesives are desirable in the construction of flushable, personal care articles such as diapers, pantiliners, training pants, adult incontinence products, etc. and in the construction of disposable health care articles such as surgical gowns and masks, wound dressing, bandages, etc.
Currently available PEO resins are not practical for melt processing, thin films, fibers or personal care applications The present inventors have developed novel PEO resins that are practical for melt processing into films and fibers. There is now a need for water-soluble adhesives that are compatible with and can be used in conjunction with these novel PEO resins. Therefore, it is an object of the present invention to provide water-soluble adhesives and a method of making such adhesives.
Unique graft copolymers of ethylene oxide are disclosed herein. The graft copolymers comprise a backbone of a polymer of ethylene oxide and side chains or branches comprising units of one or more grafted nonpolar monomers. The backbone of the graft copolymers comprises a homopolymer of ethylene oxide but may contain minor amounts of other species. Graft copolymers comprising a backbone of ethylene oxide homopolymer and grafts of nonpolar species can be exemplified by the formula 
where M and N are the nonpolar, monomeric species that are grafted to the ethylene oxide homopolymer backbone and m, n, x, y and z are positive integers. The graft copolymers of the present invention may have a minor amount of a species other than ethylene oxide in the backbone and may or may not be conterminously grafted with the nonpolar grafting species. The graft copolymers disclosed herein have been found to have properties that make them desirable as water-soluble adhesives and as components in water-soluble and water-responsive adhesives.
Water-soluble resins are desirable for flushable applications. Low molecular weight PEO resins, about 200,000 g/mol or less as determined by Theological measurements that are both thermoplastic and water-soluble resins are commercially available. However, commercially available PEO resins are difficult to process thermoplastically, particularly at higher molecular weights. Due to their poor melt strength, conventional PEO resins cannot be readily processed into films of less than 1 mil thickness even with the addition of processing aids such as plasticizers. Films produced from conventional PEO resins are too weak and too brittle for most commercial applications. A reactive-extrusion method for modifying PEO resins was developed by Wang et al. to overcome the deficiencies of commercially available PEO resins. The reactive-extrusion process developed by Wang et al. is based on the grafting of polar monomers onto PEO, to form polar grafted PEO compositions. The process developed by Wang et al. is disclosed in U.S. application Ser. No. 09/002,197, the disclosure of which is incorporated herein in its entirety. The initial experiments with polar monomers disclosed in U.S. application Ser. No. 09/002,197 establish grafting technology is a valid method to improve both the melt and solid state properties of PEO. The present invention discloses a further improvement in PEO grafting technology and includes the grafting of PEO with nonpolar monomers for water-soluble adhesive applications.
In the present invention, it was discovered that the properties of PEO change dramatically when PEO is grafted with nonpolar monomers. The change in properties is surprisingly more dramatic than in the case of grafting with polar monomers in terms of the reduction of the melt viscosity. The change in melt viscosity is so substantial that the nonpolar monomer grafted PEO resins exhibit properties desirable for hot-melt adhesives. Because PEO resins that are water soluble and can be modified by the methods described herein, the present invention provides modified PEO compositions suitable for use as water-soluble, hot-melt adhesives. Water-responsive adhesives, more particularly water-soluble adhesives, are desirable for flushable product applications but are not readily available commercially. It is demonstrated herein that nonpolar monomer grafted PEO compositions can be used as water-soluble adhesives for bonding water-responsive films, water-responsive nonwovens and water-responsive film/nonwoven laminates. The adhesive compositions comprising nonpolar monomer grafted PEO disclosed in the present invention are useful as components in flushable diapers, pantiliners, training pants, feminine pads, etc. By properly adjusting the adhesive compositions disclosed herein such as by adding tackifiers and waxes, a wide range of hot-melt adhesive compositions can be produced from the nonpolar monomer grafted PEO compositions of the present invention.